1. Field of the Invention
The present invention relates to an improved method and apparatus for aquaculture and/or mariculture which controls the water temperature and dissolved oxygen content in the water for improved harvest.
2. Description of the Prior Art
The advantages of aquaculture and/or mariculture over natural fishing have long been sufficiently apparent as to merit extensive work in this area. For example, it is clear that if food sources can be raised in controlled ponds, then the dangers of going out into the open sea will obviously be eliminated. Conventional fishing is fraught with dangers including the risk to the sailors, the possible loss of the boat, the difficulty in finding fish, the expenses of modern equipment including electronic detecting devices, marine insurance costs, labor costs, repair costs and the ever present danger of destructive weather and the hazards of marine environment on equipment. While aquaculture and/or mariculture appear to solve many of these problems, they still have not overcome all problems and, in fact, have revealed several of their own inherent problems which are not to be found in natural situations. For example, while an aquaculture and/or mariculture situation is able to control pollution, is free of predators and most destructive weather conditions, it is subject to both the difficulties of water temperature control and controlling the amount of dissolved oxygen in the water, both of which directly affect sustaining and growth of aquatic and/or marine animals.
Considering the water temperature problem first, aquaculture and/or mariculture are commonly practiced in man-made ponds located adjacent natural water supplies. The size, depth and geographic location of the ponds will have definite effects on the water temperature. If, for example, the pond is to be used to raise crustaceans, such as shrimp, then the pond preferably should have a depth of approximately two to four feet (0.61-1.22 meters). However, even at this shallow depth, the bottom layers of water may never reach the temperature of 85.degree. F. (29.4.degree. C.) which is ideal for the maximum rate of growth of shrimp. The primary difficulty lies in the fact that while the water absorbs heat from sunlight during the day, it also radiates heat from its surface during the night.
The prior attempts at overcoming this problem have included utilizing pool covers which provide a "greenhouse" effect which will absorb extra heat during the day and will slow down radiant heat escaping from the pool surface during the night. One example may be found in U.S. Pat. No. 3,998,186. This approach has worked to a certain extent, but still has the problem of temperature control in that during hot summer months the cover may cause temperatures to rise to an excessive amount and necessitate the use of fans, blowers or the like to circulate air under the cover during daylight hours to prevent the water temperature from rising too high. Other approaches to solving this temperature control problem have included the use of mechanisms to open or close the cover, which mechanisms are either manually or automatically operated in response to air or water temperature sensing devices. While to a certain extent these devices may result in proper water temperature control, they do add significantly to the cost of the operation.
Considering now the problem of dissolved oxygen, and again using shrimp as an example, if the amount of oxygen in the water drops below two parts per million, then shrimp will die. In order to have a desirable amount of growth, the shrimp require at least four parts per million of dissolved oxygen in the water. There are a number of known techniques for getting oxygen into water, such as the use of aerators. However, there are certain problems that are associated with the use of aerators, one being that they require approximately three horsepower per acre to operate, and they are primarily only surface acting in that the dissolved oxygen does not get mixed all the way to the bottom of the pond. This mixing of oxygenated water is particularly important in the aquaculture/mariculture of shrimp since shrimp are bottom dwelling animals and would need oxygen near the bottom of the pond.
At least two other factors must be considered regarding the oxygen content of the water. One is the decomposition of uneaten feed and waste accumulating on the bottom of the pond which uses up the oxygen at the bottom of the pond and causes the bottom of the pond to become foul. This can also cause the generation of harmful natural chemicals, such as hydrogen sulfide and methane, both of which are toxic to shrimp. Another factor is the algae which will be in any pond and which, when exposed to adequate sunlight, become generators of oxygen. However, sunlight does not normally penetrate more than about 24 inches (60 cm) of water so that algae below this level could be oxygen consumers rather than oxygen generators. Circulation of phytoplankton from the bottom to the surface in a continuous circulation thus becomes a desirable objective.